A multi-slice computed tomography device is also denoted hereafter as a computed tomography device or as a CT device. In spiral CT, an examination object is moved continuously through a measurement field of a recording system along a system axis of the CT device by way of a table about which the recording system with at least one radiation source and a detector simultaneously executes a multiplicity of rotations. The beam of radiation of the radiation source thereby scans the object to be examined in the shape of a spiral, and a data volume is produced which is formed from a multiplicity of three-dimensional image elements. Image reconstruction and/or image processing methods subsequently enable a two- or three-dimensional representation of at least a part of the scanned region (ROI: Region of interest) of the examination object, which is normally used to make a diagnosis.
An important variable in spiral recordings is the table feed d during a complete revolution (360°) of the recording system. The greater the table feed d selected, the quicker a region of the examination object to be examined (ROI) can be scanned. If the table feed d selected is too large compared to the detector width D used, the beam of radiation does not scan all the volume elements of the region of the examination object to be examined, and the image quality deteriorates.
The relationship between the table feed d and the detector width D used is given by the so-called dimensionless pitch or pitch factor p. The pitch or pitch factor p specifies which distance the patient table has covered during a complete revolution of the recording system with reference to the detector width used. If a multi-slice computed tomography device with a multirow detector is used, for example, for spiral recording of N spiral slices of the same width S, the pitch or the pitch factor p is given by:p=d/(N·S).  (1)
Here, N·S is the width of the detector used for recording. In the typical clinical application of the spiral scan CT devices, and in the case of multi-slice spiral CT, it is common to use spirals with a constant pitch or pitch factor p of between 0.5 and 1.5 up to a maximum of 2.
The development of ever wider, multirow detectors with an enlarged cover in the direction of the system axis of the CT device, and the achievement of ever higher rotation times TRot of the recording system has enabled the scanning times to be significantly reduced. However, this has the consequence that the speeds of the patient tables VU which are required and to be reached have not inconsiderably been increased.
If the acceleration of the patient table is not likewise increased, the problem arises that an ever larger acceleration distance is required for the table in order to bring the patient table up to the examination speed VU to be reached. The same problem arises with the slowing down of the patient table at the end of the examination, that is to say with the length of the braking distance which is required to brake the patient table to zero again from the examination speed VU.
In addition, a fan-shaped beam geometry is used in a standard spiral scan. Whereas it is sufficient in a parallel beam geometry (for example, in the case of CT devices of the first and second generations with a pencil ray beam and a partial fan beam), to record projection recordings over an angle of 180° of the examination object, in order to be able to reconstruct a complete sectional image of the examination object, it is, by contrast, necessary in a fan beam geometry to carry out the projection recording over at least an angle of 180° plus the aperture angle α of the detector in a radial direction, in order to enable the reconstruction of a complete sectional image. In such a fan geometry, detectors typically have an aperture angle from approximately 60° to 90°, as a result of which there is a need for at least a projection recordings over a total angle of at least approximately 240° to 270° in order to be able to reconstruct a complete sectional image from a projection data record.
In order to be able to reconstruct the first sectional image of a selected examination region, there is thus a need for at least one projection recording over the total angle from at least approximately 240° to 270°, specifically before the patient table has reached the position of the start of the examination region. In order to be able to reconstruct the last sectional image of a selected examination region, there is likewise a need for at least one projection recording over the total angle of at least approximately 240° to 270°, specifically after the patient table has reached the position of the end of the examination region. In the standard spiral scan, the patient table moves at the examination speed VU during the entire recording. This means that a prerun scan and a postrun scan are required once the patient table has reached an examination speed VU and there is complete patient irradiation, in order to obtain the first and last sectional images of the examination region (R.O.I.). The length of the required prerun scan and the length of the required postrun scan are proportional to the examination speed VU of the patient table and to the rotational time TRot of the recording system.
Given present patient tables with a fixed, maximum table travel distance, the consequence can be that the maximum available scanning region of the patient table is greatly reduced in some circumstances. This becomes clear, in particular, in the case of relatively wide detectors, since the length required for the prerun scan and the postrun scan increases. This becomes clear, likewise, with the relatively high examination speed VU, since the acceleration distance and the braking distance likewise increase.
In some circumstances, the facts complicate, or even render impossible the use of fastening devices fitted to the table, the fastening devices further delimiting the available scanning region, as a result of which a deterioration in image quality is to be expected. One solution of the problem is in developing new tables which make available a longer total travel range and thereby enable a larger scanning region. However, this is connected by substantial development and production costs because the entire patient container apparatus has to be reconfigured. Moreover, more room for the travel range of the patient table is thereby required in the examination space.